The structure of an active matrix OLED (organic light emitting display) or AMOLED is well known. According to FIG. 1 it comprises:                an active matrix 1 containing, for each cell (one pixel includes a red cell, a green cell and a blue cell), an association of several TFTs T1, T2 with a capacitor C connected to an OLED material. Above the TFTs the capacitor C acts as a memory component that stores a value during a part of the video frame, this value being representative of a video information to be displayed by the cell 2 during the next video frame or the next part of the video frame. The TFTs act as switches enabling the selection of the cell 2, the storage of a data in the capacitor C and the displaying by the cell 2 of a video information corresponding to the stored data;        a row or gate driver 3 that selects line by line the cells 2 of the matrix 1 in order to refresh their content;        a column or source driver 4 that delivers the data to be stored in each cell 2 of the current selected line; this component receives the video information for each cell 2; and        a digital processing unit 5 that applies required video and signal processing steps and that delivers the required control signals to the row and column drivers 3, 4.        
Actually, there are two ways for driving the OLED cells 2. In a first way, each digital video information sent by the digital processing unit 5 is converted by the column drivers 4 into a current whose amplitude is directly proportional to the video level. This current is provided to the appropriate cell 2 of the matrix 1. In a second way, the digital video information sent by the digital processing unit 5 is converted by the column drivers 4 into a voltage whose amplitude is proportional to the square of the video level. This current or voltage is provided to the appropriate cell 2 of the matrix 1.
However, in principle, an OLED is current driven so that each voltage based driven system is based on a voltage to current converter to achieve appropriate cell lighting.
From the above, it can be deduced that the row driver 3 has a quite simple function since it only has to apply a selection line by line. It is more or less a shift register. The column driver 4 represents the real active part and can be considered as a high level digital to analog converter.
The displaying of a video information with such a structure of AMOLED is symbolized in FIG. 2. The input signal is forwarded to the digital processing unit that delivers, after internal processing, a timing signal for row selection to the row driver synchronized with the data sent to the column driver 4. The data transmitted to the column driver 4 are either parallel or serial. Additionally, the column driver 4 disposes of a reference signaling delivered by a separate reference signaling device 6. This component 6 delivers a set of reference voltages in case of voltage driven circuitry or a set of reference currents in case of current driven circuitry. The highest reference is used for the white and the lowest for the smallest gray level. Then, the column driver 4 applies to the matrix cells 2 the voltage or current amplitude corresponding to the data to be displayed by the cells 2.
In order to illustrate this concept, the example of a voltage driven circuitry will be taken in the rest of this document. The driver of this example uses 8 reference voltages named V0 to V7 and the video levels are built as explained in the following table 1.
TABLE 1Gray level table from voltage driverVideo levelGrayscale voltage level 0V7 1V7 + (V6 − V7) × 9/1175 2V7 + (V6 − V7) × 32/1175 3V7 + (V6 − V7) × 76/1175 4V7 + (V6 − V7) × 141/1175 5V7 + (V6 − V7) × 224/1175 6V7 + (V6 − V7) × 321/1175 7V7 + (V6 − V7) × 425/1175 8V7 + (V6 − V7) × 529/1175 9V7 + (V6 − V7) × 630/117510V7 + (V6 − V7) × 727/117511V7 + (V6 − V7) × 820/117512V7 + (V6 − V7) × 910/117513V7 + (V6 − V7) × 998/117514V7 + (V6 − V7) × 1086/117515V616V6 + (V5 − V6) × 89/109717V6 + (V5 − V6) × 173/109718V6 + (V5 − V6) × 250/109719V6 + (V5 − V6) × 320/109720V6 + (V5 − V6) × 386/109721V6 + (V5 − V6) × 451/109722V6 + (V5 − V6) × 517/1097. . .. . .V1 + (V0 − V1) × 2278/3029251 V1 + (V0 − V1) × 2411/3029252 V1 + (V0 − V1) × 2549/3029253 V1 + (V0 − V1) × 2694/3029254 V1 + (V0 − V1) × 2851/3029255 V0
Table 1 illustrates the obtained output voltages (gray scale voltage levels) from the voltage driver for various input video levels. For instance, the reference voltages of Table 2 are used.
TABLE 2Example of voltage referencesReference VnVoltage (V)V03V12.6V22.2V31.4V40.6V50.3V60.16V70
Then, the grayscale voltage levels of following Table 3 depending on video input levels according to Table 1 and Table 2 are obtained:
TABLE 3Example of gray level voltagesVideo levelGrayscale voltage level 0 0.00 V 10.001 V 20.005 V 30.011 V 4 0.02 V 50.032 V 60.045 V 7 0.06 V 80.074 V 90.089 V100.102 V110.115 V120.128 V13 0.14 V140.153 V150.165 V160.176 V170.187 V180.196 V190.205 V200.213 V210.221 V220.229 V. . .. . .250 2.901 V251 2.919 V252 2.937 V253 2.956 V254 2.977 V255  3.00 V
As can be seen in the previous paragraph current AMOLED concepts are capable of delivering 8-bit gradation per color. This can be further enhanced by using more advanced solutions like improvements on analog sub-fields.
In any case, there will be the need in the future of displays having more video-depth. This trend can be seen in the development of transmission standards based on 10-bit color channels. At the same time, various display manufacturers like PDP makers are claiming providing displays with more than 10-bit color-depth.